1. Field of the Invention
The invention relates generally to the art of tractor-trailer frames and subframes for heavy-duty wheeled vehicles, and in particular to subframes for trailers such as van trailers. More specifically, the invention is directed to an improved slider box for a heavy-duty vehicle which includes a pair of spaced apart longitudinally extending main members. Each main member includes a pair of spaced apart generally parallel vertical walls that generally align with the spaced apart generally parallel inboard and outboard vertical walls of the hangers. The slider box utilizes hanger supports with front and rear vertical webs that align with spaced apart front and rear vertical walls of a cross member. In combination, these structural improvements allow for use of main members, cross members, hangers, supports and support members having generally thinner walls, with the slider box having improved buckle resistance, and also allow for the removal of certain cross brace members attached to the main members of the slider box, thereby reducing weight and providing improved reaction of longitudinal and side forces, yet still efficiently reacting vertical and racking forces imparted on the slider box during operation of the vehicle.
2. Background Art
Moveable subframes, typically referred to as slider boxes, slider subframes, slider undercarriages, or slider secondary frames, have been utilized on tractor-trailers or semi-trailers for many years. One or more axle/suspension systems usually are suspended from a single slider box. For purposes of clarity, hereafter, the present invention will be referred to as a slider box. It is understood that a slider box outfitted with usually two axle/suspension systems typically is referred to as a slider or slider tandem, and again, for purposes of clarity will hereinafter be referred to as a slider box. The slider box in turn is mounted on the underside of the trailer frame, and is moveable longitudinally therealong to provide a means for variable load distribution and vehicular maneuverability.
More specifically, the amount of cargo that a trailer may carry is governed by local, state and/or national road and bridge laws, and is dependent on proper load distribution. The basic principle behind most road and bridge laws is to limit the maximum load that a vehicle may carry, as well as limit the maximum load that can be supported by individual axles. A trailer having a slider box gains an advantage with respect to laws governing maximum axle loads. More particularly, proper placement of the slider box varies individual axle loads or redistributes the trailer load so that it is within legal limits. Once properly positioned, the slider box is locked in place on the underside of the trailer by a retractable pin mechanism.
Conventional or prior art slider box designs were developed before the advent of air suspension systems for trailers. At that time, leaf spring suspension systems were the suspension of choice for trailer frames and slider boxes. However, the leaf spring suspension system was unable to provide adequate load distribution for varying load situations.
Moreover, the subsequent development of air suspension systems provided load equalization among multiple axles for tractor-trailers, with or without the utilization of slider boxes, as well as improved ride quality for individual axles. Of course, the combination of a moveable slider box and an air suspension system provided maximum versatility with respect to variable load distribution and load equalization for a trailer. Unfortunately, prior art slider boxes equipped with air suspensions added unwanted weight to the trailer, primarily because those slider boxes were originally built to support spring suspensions and adapting them to incorporate air suspensions required additional bracing and support.
Vehicles containing more than one non-steerable axle, including tractor-trailers, are subject to lateral or side loads. Side loads can act through the slider box in opposite directions and the effect of such bending loads on the slider box can be significant. Moreover, a slider box is subjected to strong vertical and longitudinal loads. Thus, the loads to which the slider box is subjected must be controlled by the slider box design. Prior art slider box designs control side and longitudinal loads by utilizing rigid, and therefore heavy, main members and cross members typically made of steel. The cross members often are spaced-apart and parallel to one another, and perpendicular to the longitudinally extending elongated main members. A K-shaped cross member, as shown, described and claimed in U.S. Pat. No. 5,720,489 assigned to the same assignee, Hendrickson USA, L.L.C., as the instant invention, also have been utilized. Although the device described in the '489 patent satisfactorily performed its function, room for improvement still exists particularly with respect to maintaining or increasing the overall strength of the slider box while decreasing its weight.
Thus, within the trucking industry, reducing the weight of carrier equipment without sacrificing durability directly improves productivity by increasing the available payload that can be transported by the vehicle. Slider boxes made of steel have contributed to the excessive weight problems that have plagued slider boxes in the past. Although certain prior art slider boxes formed of steel have exhibited weight and durability improvement over other prior art steel slider boxes, such as the improvements resulting from the slider box of the '489 patent, the trucking industry continually is striving for additional optimization in slider box design. Moreover, attempts to utilize materials that are lighter than steel to construct slider boxes, such as aluminum, have been largely unsuccessful and inefficient.
Because the slider boxes having air suspensions were not connected to hanger brackets in the rear of the axles like the leaf spring suspensions that had come before them, more forces and/or loads were concentrated at the hanger brackets in front of the axles. C-shaped main members allowed for full height cross members to be used which help support the greater loads at the hanger brackets as well as the greatly concentrated loads above the air springs which were cantilevered inboard of the main members. More specifically, the increased load inputs at the hanger brackets included side loads due to drag turns that had a greater tendency to twist or rack the slider box than the prior art leaf spring suspensions. The C-shaped main member design of the slider box was conducive to incorporating angled cross members such as those shown and described in the '489 patent that effectively reacted the racking loads.
However, at least one issue in the C-shaped main member design for slider boxes became apparent when trailers turned corners too sharply causing the tires of the vehicle to impact obstacles such as lamp posts, concrete abutments, and the like. These impacts result in large longitudinal and side loads being delivered to the end of the axle of the axle/suspension system near the obstacle. These loads in turn result in considerable longitudinal and side loads being reacted at the hanger bracket. The longitudinal pulling and the sideways twisting actions on the hanger bracket could potentially result in buckling or bending of the main member of the slider box.
The present invention solves the above problems associated with prior art slider box designs by utilizing main members having spaced apart generally parallel vertical walls that generally align with the spaced apart generally parallel vertical inboard and outboard walls of the hangers. The slider box utilizes lateral hanger supports with front and rear vertical webs that generally align with spaced apart parallel vertical walls of the frame cross member. In combination, these structural improvements allow for use of main members, cross members, hangers, supports and support members having generally thinner walls with improved slider box buckle resistance. The improvements of the present invention also enable removal of additional angled cross brace members adjacent the rear axle/suspension system of the slider box, thereby reducing the weight of the slider box. The hanger support distributes side loads directly into the base member of the cross brace or cross member structure located above the hanger, allowing the hanger to react larger loads. An air spring support allows for full air spring head support substantially directly above the inboard cantilevered air spring. This air spring support provides strong support to the air spring during operation of the vehicle. Racking load support for the slider box structure is provided by the angled cross members adjacent the front axle/suspension system, thus allowing for a straight horizontal rear cross brace adjacent the rear axle/suspension system, which is able to effectively react the applicable racking loads at the rear of the slider box yet reduces weight by eliminating the need for angled cross brace members adjacent the rear axle/suspension system. A channel disposed in the main members between the front and rear pairs of hangers provides additional localized support to the slider box, thereby helping to prevent possible bending or buckling of the main members during operation of the heavy-duty vehicle and enabling further wall thickness reduction to the main members, cross members, hangers, supports and support members of the slider box. These advantages reduce the overall weight of the slider box without loss of structural integrity, but rather with an improvement thereto.